Core Principles of Thermodynamics Explained

Posted on Jul 9, 2026 in Physics

These six concepts form the foundation of thermodynamics and are essential for university examinations.

1. Open System

Definition

An open system is a system that can exchange both matter (mass) and energy (heat and work) with its surroundings.

Explanation

• Matter can enter or leave the system.
• Heat can enter or leave the system.
• Work can also be done by or on the system.

Key Transfers:

• ✅ Mass transfer: Yes
• ✅ Energy transfer: Yes

Conceptual Diagram

      Heat
       ↓
-------------------
|                 |
|   Open System   | ← Matter enters
|                 |
-------------------
       ↑
   Matter leaves

Examples

• Boiling water in an open pan (steam escapes).
• Steam turbine.
• Car engine.
• Human body (takes food and oxygen, releases waste and heat).

Applications

• Power plants
• Air compressors
• Refrigerators
• Internal combustion engines

2. Closed System

Definition

A closed system is a system that can exchange energy but cannot exchange matter with its surroundings.

Explanation

• Mass remains constant.
• Heat may enter or leave.
• Work may be done.

Key Transfers:

• ❌ Mass transfer: No
• ✅ Energy transfer: Yes

Conceptual Diagram

       Heat
        ↓
-------------------
|                 |
|  Closed System  |
|                 |
-------------------
        ↑
      Work
(No matter enters or leaves)

Examples

• Gas inside a sealed piston-cylinder.
• Pressure cooker (when tightly closed).
• Closed water bottle.

Characteristics

• Fixed amount of mass.
• Volume may change.
• Energy can cross the boundary.

3. Isolated System

Definition

An isolated system is a system that exchanges neither matter nor energy with its surroundings.

Explanation

Nothing enters or leaves the system.

Key Transfers:

• ❌ Mass transfer: No
• ❌ Energy transfer: No

Conceptual Diagram

-------------------
|                 |
| Isolated System |
|                 |
-------------------
No heat
No work
No matter

Examples

• A perfectly insulated thermos flask (ideal case).
• The entire universe (considered an isolated system).

Characteristics

• Total energy remains constant.
• Mass remains constant.
• No interaction with surroundings.

Comparison of Thermodynamic Systems

4. Internal Energy (U)

Definition

Internal energy is the total energy possessed by the molecules of a system due to their random motion and mutual interactions.

It is represented by the symbol U.

Internal Energy Components

1. Molecular kinetic energy
2. Molecular potential energy
3. Vibrational energy
4. Rotational energy
5. Electronic energy

It does not include:

• External kinetic energy
• External potential energy

Mathematical Formula

According to the First Law of Thermodynamics:

ΔU = Q – W

Where:

• ΔU = Change in internal energy
• Q = Heat supplied
• W = Work done by the system

Standard Unit

• Joule (J)

Example Calculation

A gas receives 500 J of heat and performs 200 J of work.

ΔU = 500 – 200 = 300 J

So, the internal energy increases by 300 J.

Key Characteristics

• Depends only on temperature (for an ideal gas).
• It is a state function.

5. Enthalpy (H)

Definition

Enthalpy is the total heat content of a system.

It is represented by the symbol H.

Mathematical Formula

H = U + PV

Where:

• H = Enthalpy
• U = Internal energy
• P = Pressure
• V = Volume

Change in Enthalpy

ΔH = ΔU + PΔV

At constant pressure:

ΔH = Q_p

Where Q_p is the heat supplied at constant pressure.

Standard Unit

• Joule (J)

Industrial Importance

Enthalpy is very useful in:

• Chemical reactions
• Boilers
• Steam turbines
• Heat exchangers

Practical Example

Heating water at constant pressure increases its enthalpy.

6. Entropy (S)

Definition

Entropy is the measure of the randomness (disorder) or unavailable energy in a system.

It is represented by the symbol S.

Explanation

• Greater disorder → Higher entropy.
• Lower disorder → Lower entropy.

Mathematical Formula

For a reversible process:

ΔS = Q_rev / T

Where:

• ΔS = Change in entropy
• Q_rev = Reversible heat transfer
• T = Absolute temperature (K)

Standard Unit

• J/K (Joules per Kelvin)

Examples

• Ice melts into water → Entropy increases.
• Water changes to steam → Entropy increases even more.
• Gas spreads throughout a room → Entropy increases.
• Water freezes into ice → Entropy decreases.

Key Characteristics

• Entropy always increases in a spontaneous process.
• It is a state function.
• At absolute zero (0 K), the entropy of a perfect crystal is zero (Third Law of Thermodynamics).

Summary of Thermodynamic Quantities

Essential Exam Tips

• Open System: Exchanges both mass and energy.
• Closed System: Exchanges energy only, not mass.
• Isolated System: Exchanges neither mass nor energy.
• Internal Energy: Total microscopic energy of the molecules, ΔU = Q – W.
• Enthalpy: Total heat content, H = U + PV.
• Entropy: Measure of randomness or disorder, ΔS = Q_rev / T.

These definitions, formulas, examples, and comparison points are commonly asked in B.Sc., B.Tech., and diploma thermodynamics examinations.